The present invention relates to steam turbines having seals at the horizontal midline joint between upper and lower diaphragms and particularly relates to a composite tubular woven seal for disposition between the upper and lower diaphragms, sealing the path within the diaphragms and confining the steam within the steam path.
Steam turbines typically have upper and lower diaphragms having flanges at a horizontal midline joint with seals at that location for sealing between the diaphragms. Staked keys are conventionally used at the horizontal midline joint interfaces to effect the seal therebetween. These types of metal seals typically include registering slots in the midline flanges of the upper and lower diaphragms and one or more keys extending in the slots sealing between the diaphragms. The staked keys are rigid, elongated elements formed of metal which rely on tight fits in the slots of the upper and lower diaphragms for sealing effectiveness. With these tight fits, the staked keys can often bind during assembly of the diaphragms to one another, resulting in slightly open, horizontal joints at the diaphragm interfaces, and steam leakage paths. For unbolted diaphragm assemblies, these gaps have been measured as great as 5 to 10 mils. Tight fits at the keys can also drive upper to lower axial steam face mismatches, also creating circumferential leakage paths at the steam interfaces. Because the staked keys are linear elements, sealing across a non-linear leak path requires a series of discrete keys with gaps between the keys, creating additional leakage paths. In general, mismatches between the keys and the slots resulting from a number of different causes, including manufacturing variations, have led to significant steam leakage at the diaphragm interface, with reduced machine performance. Accordingly, there is a need for an improved sealing system for the diaphragms of steam turbines at their horizontal midline joint.
In accordance with a preferred embodiment of the present invention, there is provided a steam turbine having upper and lower diaphragms, each having a horizontal flange for registration with one another and between which a composite woven tubular seal is disposed for sealing between the diaphragms. The seal is wholly compliant in nature and does not interfere with the mating of the diaphragm flanges. Also, when compressed, the seal deforms to the shape of the slot to block potential leakage paths. The seal also has flexibility to enable the seal to form different geometries of seal as necessary.
Particularly, the tubular woven seal includes an inner woven metal core, a fiber material, a metallic foil and a metal outer covering. Preferably, the inner metal core is formed of a woven stainless steel which is surrounded by silica fiber. The fiber, in turn, is surrounded by a stainless steel metal foil and the outer cover is formed of a braided material, for example, Haynes 188. Because of the resiliency of the metal core and surrounding silica fiber, the woven seal is compliant. Additionally, the metal foil layer surrounding the fiber prevents leakage between the margins of the diaphragm flanges along the horizontal midline confining the steam flow along the steam path. The braided outer covering serves as a protective wear surface. The inner metal core and silica fibers tend to retain their generally circular configuration in cross-section such that the seal, when compressed between the margins of the diaphragm flanges, is preloaded or biased for return to its circular cross-sectional configuration. Thus, any leakage flow through gaps otherwise formed at the horizontal midline of the diaphragms during steam turbine operations are sealed by the composite tubular woven seal.
In a preferred embodiment according to the present invention, there is provided a steam turbine comprising an outer shell, an inner casing within the outer shell and including upper and lower diaphragms adjoining one another along a horizontal joint midline, the upper and lower diaphragms having registering flanges at the joint midline and grooves in the flanges in registration with one another and a compliant seal partially disposed in each of the registering grooves including a seal body formed of multiple layers of different materials for compliantly sealing the registering flanges of the upper and lower diaphragms to one another along the horizontal joint midline.
In a further preferred embodiment according to the present invention, there is provided a steam turbine comprising a rotor including a plurality of circumferentially spaced buckets, an outer shell surrounding the rotor, an inner casing within the outer shell and including upper and lower diaphragms carrying a plurality of stator blades forming with the buckets a stage of the turbine in part defining a steam flowpath through the turbines, the upper and lower diaphragms adjoining one another along a horizontal joint midline and having registering flanges at the joint midline and grooves in the flanges in registration with one another, the grooves disposed radially outwardly of the flowpath and a compliant seal partially disposed in each of the registering grooves including a seal body formed of multiple layers of different materials for compliantly sealing the registering flanges of the upper and lower diaphragms to one another along the horizontal joint midline.